Move DEFAULT_PERSIST_MEMPOOL out of libbitcoinkernel

Summary:
It is no longer used by anything inside libbitcoinkernel, move it to
node/mempool_persist_args.h where it belongs.

This is a partial backport of core#25487
https://github.com/bitcoin/bitcoin/pull/25487/commits/aa306765419f7dbea12b12e15553039835ba0e4d

Depends on D15722

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15723